Breastfeeding may temporarily or permanently affect bone mass since increased calc needed for milk production may come from bone. Maximal bone mass, achieved between ages 20-35 years, is thought to relate to amount of bone mass in later life and resulting fracture risk. Determining whether breastfeeding temporarily or permanently affects bone mass may be critical for anticipating fracture risk later in life and for anticipating future impact in reproduction and lactation. The goal of the research is to determine whether there is significant loss, or gain of bone mass over a 6-month postpartum period in breastfeeding vs. bottle-feeding women. If change is observed, we want to evaluate whether that change is related to age, diet, degree of lactation, or body composition. Additionally, we want evaluate whether any observed change is likely to be a temporary "bone borrowing" or a more permanent physiologic state, and if the change is reflected in calcium content of breast milk. We will study the effect of breastfeeding on bone mass by prospectively measuring bone mass and biochemical markers of bone turnover in women of 0-1 parity who are breast- or bottle-feeding. Radial and femoral bone mass will be measured by single and dual photon absorptiometry, respectively, at parturition, and following parturition at 2 months, 4 months, 6 months, and 12 months. The study volunteers will be grouped according to three age strata: less than 20 years, 20-30 years, and greater than 30 years. Than bone mass values (and biochemical indices of bone turnover) will be compared at parturition and at 6 months between breast-and bottle-feeding women, with the 6-month values being adjusted for baseline value. Multiple regression analyses will be used to relate explanatory factors such as diet to bone mass. Repeated measures analyse will be used to examine whether any observed changes are likely to be temporary or more permanent. All analyses will take place within age strata, unless preliminary examination of the data indicates no age effect.